1. Field of the Invention
This invention relates to the field of printed wiring boards and their manufacture. More particularly, it relates to the production of double-sided printed wiring boards. The present invention relates to a method for an improved reliability surface finishing process.
2. Discussion of Related Art
In the manufacture of printed circuit boards, also called printed wiring boards, it is now commonplace to produce circuitry on both sides of a rigid or flexible substrate. Multilayer printed wiring boards consist of alternating layers of insulating substrate material and conductive metal. The exposed outer surfaces, as well as the innerlayers, of the structure are provided with circuit patterns. The most common metallurgy selected for the circuit patterns is copper.
In double-sided and multilayer PWBs, it is necessary to provide surfaces for subsequent assembly processing. The most common surface finish is the application of a solder mask onto some circuit patterns and an organic surface protection (OSP) onto others. The OSP serves two purposes; the first being to delay the oxidation of the surface metallurgy to extend the shelf-life of the panel between shipping and assembly so that additional cleaning or oxidation removal steps are prevented; the second is that the OSP improves the wettability of the metal surface for subsequent soldering operations. Other surface metallurgies can be applied to the PWB depending upon the components to be assembled. These metallurgies include gold and palladium generally with an underlying layer of nickel that provides a diffusion barrier layer between the copper and surface metal. While these metallurgies can also be used for a solderable surface, they are more suited to providing a highly conductive surface for subsequent connector systems. Of particular interest for this invention is the method for providing these additional metallurgies.
The surface finish metallurgies are generally applied by plating processes. Most common in the industry are electroless and electrolytic plating. Electroless plating is done without electricity. The advantage of an electroless process is that circuit patterns can be formed onto a PWB and immersed into the electroless solutions to deposit metal onto exposed circuit patterns. To prevent deposition on undesired areas, photolithographic processes or masking can be used.
To provide an electrolytic metal deposition, one needs a commoning layer on the surface to be metallized to provide electrical contact. If the PWB had been fabricated using electrolytic plating processes, a commoning layer is generally present, is used for all subsequent plating, and is removed during one of the final steps. A disadvantage to this method is that as dissimilar metals are applied, the final commoning layer etch exposes these dissimilar metals in contact with each other to an electrolyte solution. When this happens, a galvanic cell is generated that accelerates the etch rate of the less noble metal. In this case, the circuit pattern would etch faster than the surface finish metal. This can produce an undercut of the surface metallurgy causing it to produce metal overhanging the underlying circuit pattern, which can flake off and contact adjacent circuit patterns causing an electrical short. The industry refers to this phenomenon as “slivering”.
For a PWB fabricated using an electrolytic double-sided plating process, a commoning layer is provided on both surfaces of the PWB substrate. In the industry, this method is performed by first electrolytically pattern plating a first copper layer over a commoning layer. A photoresist is then applied after the commoning layer is formed. While leaving the pattern photoresist on the panel, subsequent electrolytic metallizations can take place. However, in some operations, the process is a full panel plate with subtractive etching to form the circuit traces. As a result, there is no commoning layer on the panel's surfaces as the copper is etched to the laminate substrate surface.
The two primary methods in the industry for pattern plating are pattern electroplate and pattern electroless plating.
In pattern electroplate, holes are drilled through or into a copper clad dielectric substrate to form through holes (extending from one planar surface of the substrate to the other) or vias (extending in from one surface but terminating within the substrate) followed by plating a commoning layer of copper onto the surface of the board. A commoning layer is an electroless copper applied over a PdSn catalyst layer. For one such process, the substrate is copper clad on at least one of its surfaces and the process employed would be a subtractive process whereby the unwanted copper is removed by chemical etching or laser ablation. The substrate typically is nonconductive, and may be provided with through holes or vias.
Because the holes extend into or through the dielectric, which is an insulator, a continuous electrical connection is first created by putting a thin metallization of copper in the holes. A photoresist is applied over the commoning layer and a pattern is formed in the photoresist through known photolithographic techniques (expose, develop) such that the open channels in the photoresist are the places where circuit traces will be formed. The substrate is then electroplated with copper, and metallization build-up only occurs in the open channels to form circuit traces. A layer of tin/lead is plated over the copper traces, followed by stripping of the photoresist and etching of the commoning layer with the copper traces protected by the tin/lead during the etch. Then the tin/lead is etched off. The result is a PWB with copper circuit traces onto which a solder mask is later placed.
It should be observed that the copper clad substrate can be any thickness, limited only to the handling capability of the fabricator. Very thin cores (approx. 5-8 mils) with laser drilled holes and vias up to multilayer boards (MLB) composites or 300+ mils and mechanically drilled holes and vias can be processed if equipment permits. The commoning layer is generally thin, about 100 microinches, and is usually applied with an electroless strike. It can also be applied by sputter coating. The photoresist can be any of the many available for copper plating in the industry. Likewise, the expose and develop chemistries are industry known. The subsequent copper plating can be direct current (DC) or pulse plated. Pulse plating provides the advantage of a more even distribution, less thieving area needed and better ‘throw’ into the drilled holes. DC is the most common, but has the drawbacks of non-uniform plating on isolated features and not very good throw in high aspect ratio (length to diameter) holes. Once the panel has been pattern electroplated, the commoning layer is still present under the photoresist and opens up several finishing options:
1. If tin/lead (solder) is desired on certain areas of the board, then prior to etching off the tin/lead in the last step, a photoresist/mask is applied to protect those areas during the tin/lead etch process. The mask is then removed
2. If gold is desired, for instance, on the land grid array pads, then after copper plate and tin/lead plate, a second photoresist/mask is applied, leaving open the features to be gold plated bearing in mind that the first pattern plate resist is still on the board. The tin/lead is etched off the pads, leaving a copper surface. Then the board is electrolytically nickel/gold plated, the second photomask is stripped, the first photomask is stripped either simultaneously or subsequently, the commoning layer is etched off, creating the galvanic cell phenomenon seen by the industry as slivering, and then the tin/lead is etched off.
3. These steps can be extended further. For example, if tin/lead areas are needed on a board with gold, another photomask operation is carried out. If there are requirements that the gold be thick in one place and thin in another, again, masking and plating operations are performed as long as the base commoning layer is still present.
A similar pattern plate option is practiced called ‘semi-additive’, the difference being that, instead of an electrolytic plating to create the traces in the patterned resist, a full-build electroless process is used. Since the commoning layer is still present, all of the aforementioned finishing processes can be carried out.
Still another pattern plate process is a full-build electroless process. In this process, there is no base copper clad panel and no commoning layer. The substrate surface is catalyzed with a palladium seed, and the photoresist is applied directly to this surface and patterned. The electroless plating builds up additively to create the traces. Since there is no commoning layer, if it is desired to continue with precious metal or tin/lead plating, this plating also has to be electroless.
Electroless gold is advantageous for wire bonding, but is not good for LGA pads or connector fingers where a ‘hard’ gold is needed. Solder paste can be used for places that need tin/lead.
The other method to create a PWB is the standard ‘print and etch’ process. In this process, the drilled substrate is plated with copper, a negative photoresist is applied and patterned, the panel is then etched such that the areas opened in the photoresist are etched away and the covered areas are protected. Again, there is no commoning layer, so to finish a board with hard gold requires a method, such as personalizing one side of the substrate only, so that the other side is left copper clad. The copper clad side acts as a commoning layer for a single sided gold plate. The gold plated side is then subsequently covered with a photoresist and the copper clad side is patterned and etched.
The problem with the first ‘base process’ is that the nickel and gold are plated onto the copper with the photoresist in place. Hence, the process is ‘non-conformal’ in that it does not plate the sidewalls of the copper features. When the commoning layer is etched, the more noble nickel/gold layer will not be etched but the underlying copper features and commoning layer will. When two dissimilar metals are in contact with one another, a galvanic cell is created upon exposure to an ionic solution. In this galvanic cell, the less noble metal (copper) acts like the anode and is preferentially etched while the more noble metals (nickel and gold) are the cathode. This is the same analogy as the rivets in the hull of a ship. If the rivets are a different metal than that of a ship hull, the rivets will corrode when exposed to seawater. In the circuit board, the galvanic etch acts to undercut the copper feature leaving a nickel/gold overhang which can subsequently break off and form a ‘sliver’. This sliver of conductive metal can then cause shorting of the circuits on the PWB.
Various techniques for PWB manufacture are described in the following patents.
U.S. Pat. No. 4,790,912 describes a selective plating process where, after a photolithographic process produces electrical tracings of copper, then tin/lead alloys may be coated upon at least part of the copper tracings.
U.S. Pat. No. 5,028,513 describes a process for producing a printed circuit board by pattern plating wherein, after copper traces are formed, additional metal such as solder, copper, nickel, gold, or other metal is deposited on the tracings.
U.S. Pat. No. 5,262,041 describes additive plating wherein a flash metal is deposited onto a substrate, followed by application of a resist. Windows are left open in the resist to the flash metal after which additional metal is deposited within the windows.
U.S. Pat. No. 5,985,124 describes nickel or nickel alloy electroplating wherein at least a partially masked electrical circuit has nickel metal deposited hereon. This may be followed by plating with gold or other precious metal.
U.S. Pat. No. 6,221,229 describes a method for forming metal conductor patterns. After metal circuit lines are formed and any excess metal is removed by laser ablation, additional metals are electrolytically deposited onto at least a portion of the circuit lines.